In some air-conditioning apparatuses such as a multi-air-conditioning apparatus for buildings, a heat source unit (an outdoor unit) is disposed outside a structure, and an indoor unit is disposed in a room of the structure. Refrigerant circulating in a refrigerant circuit of such an air-conditioning apparatus dissipates (or absorbs) heat to/from air supplied to a heat exchanger of the indoor unit, and heats or cools the air. The indoor unit sends the heated or cooled air to an air-conditioned space, thereby heating or cooling an interior space (the air-conditioned space).
Since a building generally includes a plurality of interior spaces separated from one another by, for example, walls, the air-conditioning apparatus also includes a plurality of indoor units. For a large building, refrigerant pipes connecting the outdoor unit and the indoor units are 100 m in length in some cases. Such a large length of the pipes connecting the outdoor unit and the indoor units increases the amount of refrigerant with which a refrigerant circuit is charged accordingly.
An indoor unit of such a multi-air-conditioning apparatus for buildings is generally used while being disposed in an interior space (e.g., an office, a living room, or a store) where a person is present. When refrigerant leaks from an indoor unit disposed in the interior space for some reasons, this leakage might cause problems with respect to its influence on the human body and safety because some types of refrigerants are flammable and/or toxic. Even a leakage of refrigerant that is not harmful to the human body might cause a decrease in oxygen concentration in the interior space and affect the human body.
To solve such problems as described above, an air-conditioning apparatus of a proposed technique employs a secondary loop system. Specifically, the secondary loop system is used for air-conditioning an interior space where a human is present by including a primary loop serving as a refrigerant circuit in which refrigerant circulates and a secondary loop serving as a heat medium circuit in which an unharmful heat medium such as water or brine circulates (see, for example, Patent Literature 1).